There are some gaps regarding the characters’ motivations. The person giving the spoilers was kind of cryptic about it, but basically, everything that’s happening was orchestrated by Hanabi’s sister. Both Kirigaya raping and sex-trafficking Hanabi, and Nanao drugging and attempting to rape the MC (she tried, but his penis didn’t get hard).
As for why Hanabi’s sister would set up these heinous acts, even against her own sister, that’s where it gets foggy. She’s messed up in the head because of trauma related to their father. The spoiler source said their father didn’t rape Hanabi’s sister, but whatever he did caused her to develop a twisted love-hate relationship with Hanabi.
As for consequences… there are basically none for anyone. Hanabi’s sister just gets a warning that if she tries any more crap like that, they’ll tell their father. Nanao is fired, and that’s it. And Kirigaya will get beaten up by the MC at some point, but that’s the maximum punishment he’ll get.